Probably cheaper to buy a used fretless neck. As far as new is concerned, expect to pay about the same for a good lined fretless neck, as you would for having a fretted neck converted by a competent luthier. Despite what a lot of self-appointed experts might think, it's not easy to do it right.
Perhaps look in Warmoth's showcase...I've seen them there from time to time.
I'm sure it'd fit the pocket, but I don't know if the authorities would cite me for putting a P neck on a J bass. :drill: